In many stitchings to be carried out by guiding a fabric to a needle, a blindstitching is typical, and the invention will be explained with reference to this stitching.
As shown in FIG. 7 and as generally known, a blindstitched pattern has a predetermined width W between a left needle dropping position L and a right needle dropping position R. Further it is generally known that the left needle dropping position L is a reference along which is guided an end part of the upper fold of fabric to be sewn as shown in FIG. 8.
It is however required that the left and right needle dropping positions L, R are changed in dependence upon the thickness of a fabric. So far it has been done as follows:
In FIGS. 9a to 9c, a basic line of needle position is shown by "X" which corresponds to one of the L, M and R needle positions as generally known. If the basic line X is set to the left needle dropping position L as shown in FIG. 9(a), the left needle dropping position L is fixed, and the right needle dropping position R is variable as shown at R1, R2, R3 relative to the left needle dropping position L.
If the basic line X is set to the right needle dropping position R as shown in FIG. 9(b), the right needle dropping position R is fixed, and the left needle dropping position L is variable as shown at L1, L2, L3 relative to the right needle dropping position R.
If the basic line X is set to the middle needle position M as shown in FIG. 9(c), the left and right needle dropping positions L, R are varied at the same ratio with respect to the middle position M.
The fabric F is folded in the manner as shown in FIGS. 10(a) to 10(c) to provide therein an upper fold F7, and a lower fold F2 to be blind-stitched thereat, and the edge of the upper fold F1 is set laterally to a guide G of a blind stitching presser foot 47 as shown in Figs.8, 13, 16 and 17, in which the presser foot 47 is pressed against the upper and lower folds F1, F2 of the fabric F and the guide G is pressed against the lower fold F2. The guide G may be fixed to the presser foot 7 as shown in FIG. 13 or may be adjustable laterally of the presser foot 47 as shown in Figs.16 and 17.
Then it is required that the left needle dropping position L is set to an appropriate position on the fabric F where a needle penetrates the upper and lower folds F1, F2 of the fabric F as shown in FIG. 8. More precisely the appropriate position is represented by (a) in FIG. 11, the position being slightly spaced inward from the edge of the upper fold F, so that the formed stitches S may not be conspicuous as shown in FIG. 12(a) when the uppermost part of the fabric F is unfolded. The stitches S in FIG. 12(a) are shown in contrast to the conspicuous stitches S1 (FIG. 12b) which are undesirous and may be formed when the left needle dropping position L is set to the position (b) on the fabric F in FIG. 11 where the needle is spaced far away inward from the edge of the upper fold F1, as compared with the needle position (a). Moreover such conspicuous stitches S1 will cause shrinkage of the fabric when the latter is unfolded. On the other hand, if the left needle dropping position L is set to a position (c) in FIG. 11 where the needle is spaced outward from the edge of the upper fold F1, the blindstitches are not formed on the upper fold F1 and result in failure of the blindstitching function as shown in FIG. 12(c).
With respect to the formation of blindstitches, there have been generally three types of operation as follows.
(1) A blindstitching presser foot having a guide fixed thereto is employed in combination with a sewing machine storing a blindstitch pattern of a predetermined amplitude.
(2) A blindstitching presser foot having a guide fixed thereto is employed in combination with a sewing machine storing a blindstitch pattern, the amplitude and basic line of which may be varied.
(3) A blindstitching presser foot having a guide provided as being adjustable laterally thereof is employed in combination with a sewing machine storing a bilndstitch pattern, at least the amplitude of which may be varied.
In case of item (1), the blindstitching presser 47 having the guide G fixed thereto as shown in FIG. 13 is employed in combination with the sewing machine as mentioned in the item (1). In this case, the needle positions are constant at both sides of the guide G. However because of the possible divergences in precision of parts in each of the sewing machine, generally the guide G is mounted on the presser foot 47 such that the needle may drop at the left position L where the needle is sufficiently spaced inward from the edge of the upper fold F1 to secure the correct formation of blindstitches. More precisely, the dimension (d) in FIG. 8 is amplified more than normally required, and this often causes the conspicuous (large) stitches S1 which are undesired.
In case of item (2), the blindstitches may be produced in the manner as shown in FIGS. 9(a), 9(b) or 9(c), in which the amplitude may be varied with respect to the basic line as shown at L, R or M as described herein above. In case of FIG. 9(a), the right needle dropping position R may be variably set from the fixed left needle dropping position L. However as the distance (d) in FIG. 8 is predetermined by the guide G with an amplitude dimension, for the reason as mentioned above, this may cause conspicuous stitches S1. In case of FIG. 9(b), the left needle droping position L may be variably set from the fixed right needle dropping position R. However as distance (d) in FIG. 8 is predetermined by the guide G with an amplitude dimension, this may cause the lower fold F2 to loose by of a considerable size of dimension E to the left loose as shown in FIGS. 14 and 15. In case of FIG. 9(c), if one of the left and right needle dropping positions L, R, the other the needle dropping positions is varied, and therefore the defects as mentioned in item 1 and in case of FIG. 9(b) may be caused.
In case of item (3), the guide G may be adjusted laterally of the presser foot 47 as shown in FIGS. 16 and 17 to variably determine the distance (d) of the blindstitch in FIG. 8. However the mechanical play of the presser foot structure will fail to stabilize the guide G in the set position relative to the presser foot 47 resulting in divergences of the blindstitches formed on the fabric.
The above mentioned conventional mode of stitching is insufficient with respect to required conditions of the formation of blindstitches, due to the defects as described above since it has been desired that,
(1) the fabric guiding parts are stable;
(2) the darning amplitude is made as small as possible; and
(3) the needle dropping position may be adjusted with respect to the fabric guiding.